JPH0653749A - Broad band amplifier - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a wide-band amplifier which is used in a high-bandwidth input amplification stage such as an oscilloscope, has little fluctuation due to temperature, and does not deteriorate the amplification characteristic. [Structure] Voltage control means (6, R ₃ , R ₄ , R ₅ , R ₆ , R
₉ ) outputs to the variable resistance unit 2 a control voltage proportional to the error between the input detection signal of the input terminal Ti of the field follower 1 of the source follower and the output detection signal of the output side transistor 5. Although the variable resistance portion 2 changes its resistance value according to the control voltage, the drain current from the field effect transistor flowing through the variable resistance portion is always kept constant by the constant current source 3. Therefore, the output voltage of the output side transistor fluctuates according to the error, and the input detection voltage and the output detection voltage are controlled to be always equal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide band amplifier used in a wide band input amplifying stage such as an oscilloscope, and more particularly to a wide band amplifier capable of compensating for influences of variations in element characteristics and drift due to temperature change.

[0002]

2. Description of the Related Art Conventionally, a wide band amplifier used in an analog electronic measuring instrument such as an oscilloscope needs to amplify a wide frequency band signal ranging from direct current to high frequency with low drift. In particular, when used in a vertical input circuit such as an oscilloscope, it has a high input impedance to reduce the load effect on the signal source under test, and a low output impedance to widen the bandwidth of the subsequent amplifier. A broadband amplifier is needed. Further, it is required to realize a stable wideband amplifier that the variation in element characteristics or the drift due to temperature change is small.

Conventionally, as in the inventions disclosed in, for example, Japanese Patent Publication No. 61-1926 and Japanese Utility Model Publication No. 61-15622, an input signal is divided into a high-frequency signal component and a low-frequency signal component, and the amplified signal is amplified. A method of adding both signals is known. In this method, a low-frequency signal and a high-frequency signal are separated and amplified by separate amplifiers, respectively, to obtain stable broadband amplification. However, a self-bias must be applied to the field-effect transistor used in this method, and this self-bias resistor requires a high resistance value, which causes thermal noise in the resistor. There was a problem that the output waveform was distorted.

In order to solve this problem, Japanese Patent Publication No.
As disclosed in Japanese Patent Laid-Open No. 3-15764, there is known a wide band amplifier which uses a field effect transistor of a source follower and which is stable against an influence due to variations in characteristics of elements and a drift due to variation in ambient temperature. There is. This wide band amplifier will be briefly described with reference to FIG. In FIG. 5, the gate and drain of the field effect transistor 31 are connected to an input terminal 32 and a positive voltage source, respectively, and the source is connected to a negative voltage source via a constant current source composed of a bipolar transistor 33 and a resistor 34. . The source of the field effect transistor 31 is further connected to the base of the transistor 35. Voltage divider 36 provided at the input terminal 32 and voltage divider 37 provided at the output side
Have substantially equal voltage division ratios, and the divided input and output signals are supplied to the operational amplifier 38. Operational amplifier 3
The error signal 8 is supplied to the base of the transistor 33 through the low pass filter 39 including a resistor and a capacitor.

In FIG. 5, an input signal applied to the input terminal 32 is amplified by a wide band amplifier having a field effect transistor 31 and a transistor 35 (the degree of amplification is approximately 1). The signal path including the pre-input voltage divider 36, the output voltage divider 37 and the operational amplifier 38 maintains the circuit stability and accuracy of the wide band amplifier at low frequencies. The operational amplifier 38 compares a part of the signal from the input voltage divider 36 with a part of the output signal from the output voltage divider 37 and applies an error signal to the base of the transistor 33. When the gate-source voltage Vgs of the field-effect transistor 31 or the base-emitter voltage V _{BE of the} transistor 33 changes due to the difference in operating conditions and the output voltage differs from the input voltage, a part of the output signal of the operational amplifier 38 is generated. Return to 38,
The difference in output and input voltage is automatically corrected by the high gain operational amplifier 38 and current source transistor 33. That is, the output voltage is modified to be equal to automatically input voltage regardless of the change in Vgs and V _BE.

[0006]

In the wide band amplifier shown in FIG. 5, circuit stabilization in the low frequency band is performed by changing the drain current Id and the gate of the field effect transistor 31 of the source follower.
This is realized by changing the source voltage V _gs .
Therefore, there is no need for a self-biasing resistor, and there is no problem due to thermal noise. However, the transconductance g _m of the transfer characteristic of the field effect transistor 31 changes due to the change of the emitter current flowing through the transistor 33, in other words, the drain current Id of the field effect transistor 31. The change in the mutual conductance g _m affects the amplification characteristic in the high frequency band of the field effect transistor 31 of the source follower.

An object of the present invention is to provide a wide band amplifier which has a function of correcting a drift or the like generated in an internal element and which does not deteriorate the amplification characteristic in a high frequency band.

[0008]

A broadband amplifier of the present invention comprises a field follower field effect transistor connected to an input terminal and a constant current for connecting a constant source current to the source side of the field effect transistor. Source, an output terminal of the constant current source and an input terminal of the field effect transistor, the voltage variable resistance element changing resistance according to the magnitude of the control voltage applied, and the output terminal of the voltage variable resistance element. Output-side transistor to which the voltage signal is supplied, and voltage control means for applying an error between the output detection voltage of the output-side transistor and the input detection voltage of the input terminal as a control voltage to the voltage variable resistance element. The input detection voltage and the output detection voltage are controlled to be substantially equal to each other. In the second invention, a variable gain amplifier that amplifies a voltage signal at an output end of a voltage variable resistance element and applies the amplified voltage signal to the output side transistor, and a level of an output detection voltage of the output side transistor of the variable gain amplifier are provided. The output voltage adjusting means adjusts the gain in conjunction with the switching.

In the third invention, the input detection voltage is generated by a voltage dividing resistor connected to the input end of the field effect transistor, and the output detection voltage is connected to the output end of the output side transistor. It is generated by a voltage dividing resistor.

[0010]

The voltage control means outputs to the voltage variable resistance element a control voltage proportional to the error between the input detection signal at the input end of the field effect transistor of the source follower and the output detection signal of the output side transistor. Although the voltage variable resistance element changes its resistance value according to the control voltage, the source current from the field effect transistor flowing to the element is maintained constant. As a result, the base voltage applied to the output side transistor fluctuates according to the change of the voltage variable resistance element, the output voltage of the transistor fluctuates according to the error, and the input detection voltage and the output detection voltage are adjusted to be equal. To be done.

Further, the voltage signal at the output end of the voltage variable resistance element is amplified by the variable gain amplifying means before being applied to the output side transistor, and the detection level of the output detection signal fed back to the voltage control means is adjusted. Thus, the level of the output signal of the output side transistor is adjusted.

[0012]

1 shows an embodiment of a wide band amplifier according to the present invention. As shown by the broken line in FIG. 1, the wide band amplifier is generally composed of a high input impedance section A, a low output impedance section B, and a compensation circuit section C. The high input impedance section A comprises a field follower field effect transistor 1, and an input signal is input to the gate terminal (input terminal Ti) of this field effect transistor 1. The variable resistance section 2 and the constant current source 3 are sequentially connected in series to the source terminal of the field effect transistor 1.
The constant current source 3 can have a well-known circuit configuration in which a constant current is always supplied. The base of the pnp transistor 4 is connected to the connection point between the output end of the variable resistance unit 2 and the input end of the constant current source 3. A negative voltage is applied to the collector of the pnp transistor 4, the base of the npn transistor 5 is connected to the emitter of the pnp transistor 4, and the positive power source is connected via the resistor R ₁ . The pnp transistor 4 is
The level of the bias voltage V _be2 applied to the base of the npn transistor 5 is adjusted. The collector of the npn transistor 5 is connected to the positive power supply, the emitter terminal thereof is connected to the negative power supply via the resistor R ₂ , and the output voltage is taken out therefrom as the output terminal To.
The low output impedance section B is mainly composed of a pnp transistor 4 and an npn transistor 5.

Voltage dividing resistors R ₃ and R ₄ are connected in series between the output terminal To and the ground. This voltage dividing resistor R ₃
An output detection voltage is taken out from the intermediate connection point of R ₄ and R ₄ and is applied to the negative side input terminal of the operational differential amplifier 6. On the other hand, voltage dividing resistors R ₅ and R ₆ are connected between the input terminal Ti and the ground. An input detection voltage is taken out from an intermediate connection point of the voltage dividing resistors R ₅ and R ₆ and given to the positive side input end of the operational differential amplifier 6. The operational differential amplifier 6 amplifies the difference between the two detection voltages and applies a control voltage to the variable resistance section 2 via the resistor R ₇ . A capacitor C ₁ is connected to the operational differential amplifier 6 to mainly amplify low frequency components. The compensation circuit section C mainly includes a variable resistance section 2, a constant current source 3, and a voltage dividing resistance R.
₃ and R ₄ , voltage dividing resistors R ₅ and R ₆ , and an operational differential amplifier 6.

The variable resistance section 2 comprises a field effect transistor 21, a resistor R ₈ and a capacitor C ₂ connected in parallel between the source and drain of the field effect transistor 21. The resistor R ₈ is a shunt resistor that is shunted with the channel resistor R _DS of the field effect transistor 21 in order to cause the drain current of the field effect transistor 1 to flow to the constant current source 3. Further, the capacitor C ₂ is provided for high frequency characteristic compensation in order to pass a high frequency signal. A control voltage from the operational differential amplifier 6 is applied to the gate of the field effect transistor 21. FIG. 2 shows an example of the relationship between the gate-source voltage V _GS and the drain-source resistance R _DS of the field effect transistor 21, and as the gate-source voltage V _GS increases, the channel resistance R _DS increases. It shows that you do. Then, the field effect transistor 2
When the control voltage is applied to the gate of No. 1, the gate-source voltage V _GS also changes in accordance with the change in the control voltage.
Its resistance R _DS also varies. In this way, the field effect transistor 21 operates as a voltage control variable resistance element that changes the resistance between the source and drain according to the control voltage.

Next, the operation of the wide band amplifier shown in FIG. 1 will be described. For convenience of explanation, the variable resistance portion 2 will be replaced with a variable resistance element R whose resistance value is changed by a control voltage. The input voltage applied to the input terminal Ti is V _i, and at this time, the gate of the field effect transistor 1
The voltage between sources is V _gs . The gate-source voltage V _gs is determined by the temperature-dependent drain current and the pinch-off voltage. Due to the presence of the constant current source 3, the input voltage V
Field effect transistor 1 regardless of the level of _i
Since the drain current Id, that is, the current flowing into the constant current source 3 is constant, a voltage drop of R · Id occurs between the source terminal of the field effect transistor 1 and the input terminal of the constant current source 3. Therefore, the base of the pnp transistor 4 has (V _i
A voltage of −V _gs −R · Id) will be applied. At this time, _assuming that the base-emitter voltage of the pnp transistor 4 is V _be1 , the base-emitter voltage of the npn transistor 5 is V _be2, and the output voltage of the output terminal To is V _o , the output voltage V _o can be expressed by equation (1).

[0016]

[Number 1] V _O = V _i but -V _gs -R · Id + V _be1 object of -V _be2 (1) The present invention is to a V _o = V _i, V _gs as a drift factor, V _be1 and V _be2 exists. Therefore, the following expression (2) is obtained from the expression (1).

## EQU2 ## −V _gs −R · Id + V _be1 −V _be2 = 0 (2) If the formula (2) is established, the drift generated in the internal element of the wide band amplifier can be canceled. The equation (2) is transformed into the following equation (3).

R = (− V _gs + V _be1 −V _be2 ) / Id (3) Here, the variable resistance element R is a combined resistance of the channel resistance R _DS of the field effect transistor 21 and the resistance R ₈ .
It is expressed by the following equation (4).

[Number 4] _{R = (R DS · R 8} ) / (R DS + R 8) (4) (3) equation (4) from the following equation (5) is obtained.

[Number 5] _{(R DS · R 8) /} (R DS + R 8) = (- V gs + V be1 -V be2) / Id (5) here, V _gs, V _{be1, Vbe2} drift with temperature change or the like In this case, the resistor R ₈ is appropriately selected and the gate-source voltage of the field effect transistor 21 is controlled as shown in FIG. 2 to change the channel resistance R _DS so that the equation (5) is satisfied. Then, the output voltage V _O is always equal to the input voltage V _i .

When a drift appears in the output voltage V _o due to a change in ambient temperature or the like, the output detection voltage fluctuates in accordance with the drift, and the differential amplifier 6 amplifies an error from the input detection voltage to produce a field effect transistor as a control voltage. 21 is applied to the gate. The resistance R _DS between the source and drain changes as described above according to the change in the control voltage.
The drain current Id of the field effect transistor 1 is the resistance R _DS
Of the pnp transistor 4 changes as the resistance R _DS changes, and as a result, the base-emitter voltages V _be1 and V _{be2 of} the pnp transistor 4 and the npn transistor 5 change. Then, the drift of the output voltage V _O is eliminated. In this way, the compensation circuit section C operates so that the input detection voltage and the output detection voltage always match, so that the output voltage V _O does not drift. Furthermore, since the drain current Id of the field effect transistor 1 is always held at a constant value,
The transconductance of the field effect transistor 1 is also kept substantially constant, and there is no inconvenience that the operating point changes and the output waveform is distorted.

In the wideband amplifier embodiment of FIG. 1,
Although the field effect transistor 21 is used as the variable resistance element provided in the variable resistance part 2, the CdS opto-isolator 24 may be used instead of the field effect transistor 21 as shown in FIG. In the embodiment of FIG.
The control voltage output from the differential amplifier 6 is CdS
It is applied to the photodiode 241 of the isolator 24. Light having an intensity proportional to the control voltage is output from the photodiode 241 and is irradiated on the CdS 242, and the resistance value is changed according to the light intensity. As a result, CdS
The isolator 24 functions as a variable resistance element, and a voltage drop R · Id (R is an equivalent resistance of the variable resistance section 2) is generated by the inflowing drain current Id. The configuration and operation other than the variable resistance unit 2 are the same as in the case of FIG.

FIG. 4 shows another embodiment of the present invention. The same parts as those in FIG. Figure 4
The difference between the wide band amplifier of FIG.
That is, the variable amplification section D is connected between the intermediate connection point between the constant current source 3 and the low output impedance section B. The variable amplification section D is composed of an operational amplifier 7 whose gain can be changed. The low output impedance section B is composed of only the npn transistor 5, and the output of the operational amplifier 7 is directly applied to the base of the transistor 5. A resistor R ₁₁ for determining the gain is connected to the operational amplifier 7, and when the interlocking switch 8 is turned on, the resistor R ₁₂ is connected in parallel with the resistor R ₁₁ . When the interlocking switch 8 is turned on, the voltage dividing resistor R ₉ is connected in parallel with the voltage dividing resistor R ₄ on the output side. In this example, the gain of the operational amplifier 7 is 2 when the interlock switch 8 is open, but the gain is set to 10 when the interlock switch 8 is turned on. In the wide band amplifier of FIG. 4, the relationship between the input voltage V _i and the output voltage V _O is expressed by the following equation (6), where G is the gain of the operational amplifier 7.

[0020]

[6] _{V O = (V i -V gs} -R · Id) · While G-V _be2 (6) It is an object of the invention to _{V O = G · V i,} V gs as a drift factor , V _be2 and G are present. Therefore, equation (7) is obtained from equation (6).

(−V _gs −R · Id) · G−V _be2 = 0 (7) If the formula (7) is satisfied, the drift generated in the internal element of the wide band amplifier can be canceled. Equation (8) is obtained by modifying equation (7).

R = (− G · V _gs −V _be2 ) / (G · Id) (8) The following formula (9) is obtained from the formulas (4) and (8).

[ _Equation 9] R _DS · R ₈ = (− G · V _gs −V _be2 ) / (G · Id) (9) Therefore, as in the case of FIG.
By controlling the gate-source voltage of 1 to change the channel resistance R _DS to satisfy the expression (9), the input voltage V _i and the output voltage V _O can be made equal. Also in the case of FIG. 4, since the drain current Id of the field effect transistor 1 is always constant, the mutual conductance is also constant, and the output voltage is not distorted. The output detection voltage when the interlock switch is turned on is R ₄ = R ₄ · R ₉ /
It can be obtained by substituting (R ₄ + R ₉ ).

[0021]

According to the present invention, since the drain current flowing through the field effect transistor of the source follower used as a high input impedance circuit can be constantly maintained at a constant value, the mutual conductance of the field effect transistor is maintained at a substantially constant value. Therefore, the amplification characteristic of the wide band amplifier is not deteriorated. Further, the drift of the output voltage caused by the change of the ambient temperature is automatically compensated, and stable amplification can be realized without being influenced by the variation of the element characteristics and the change of the operating point due to the temperature. In particular, the present invention, which has extremely stable amplification characteristics, is suitable as the first stage amplifier of an oscilloscope that measures a signal in the frequency band from DC to approximately 250 MHz.

Further, the present invention can realize an amplifier with less drift even when a variable gain amplifier is connected.
In particular, the vertical axis sensitivity can be easily switched by using it in the input amplification stage of the oscilloscope.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of a wideband amplifier of the present invention.

FIG. 2 is a characteristic diagram showing the relationship between the gate-source voltage and the drain-source resistance of a field effect transistor.

FIG. 3 is a diagram showing another embodiment of the variable resistance element provided in the voltage control unit of FIG.

FIG. 4 is a circuit configuration diagram showing another embodiment of the wide band power amplifier of the present invention.

FIG. 5 is a circuit diagram of a conventional wide band amplifier.

[Explanation of symbols]

First field effect transistor 2 variable resistance part 3 the constant current source 5 npn transistor 6 operational differential amplifier 7 operational amplifier 8 interlock switch _{R 3, R 4, R 5} , R 6, R 9 dividing resistor A high input impedance section B Low Output impedance C Compensation circuit D Variable amplifier

Claims (3)

[Claims] 1. A field effect transistor of a source follower connected to an input terminal, a constant current source connected to the source side of the field effect transistor for supplying a constant source current, and an output terminal for the constant current source. A voltage variable resistance element having an input terminal connected to the source of the field effect transistor and changing its resistance according to the magnitude of a control voltage applied; and an output side transistor to which a voltage signal at the output terminal of the voltage variable resistance element is supplied. A voltage control means for applying an error between the output detection voltage of the output side transistor and the input detection voltage of the input terminal as a control voltage to the voltage variable resistance element, wherein the input detection voltage and the output detection voltage are substantially equal to each other. A broadband amplifier characterized by being controlled to be equal. 2. A variable gain amplifier that amplifies a voltage signal at an output end of the voltage variable resistance element and applies the amplified voltage signal to the output side transistor, and a level of an output detection voltage of the output side transistor is switched between gains of the variable gain amplifier. 2. The wide band amplifier according to claim 1, further comprising an output voltage adjusting unit that adjusts in conjunction with the output voltage adjusting unit. 3. The input detection voltage is generated by a voltage dividing resistor connected to the input terminal of the field effect transistor, and the output detection voltage is generated by a voltage dividing resistor connected to the output terminal of the output side transistor. The wideband amplifier according to claim 1 or 2, wherein